The present invention relates generally to memory cells and more particularly to a memory cell capable of operation in a read/write and a read only memory configuration.
Many process oriented control applications in todays industries are using low cost micro-controllers. Such systems require a limited amount of memory. Most applications require the incorporation of both pre-programmed memories and real time read/write memories. A predetermined sequence of events or programs are stored in non-volatile or preprogrammed memories while data relevant to a particular point in time or alterable information is stored in programmable memories. Because of a limited amount of memory available, memories capable of operation as non-volatile memories and programmable memories were developed.
Prior art dual functional memories included extensive metal paths and switches to preselect the state of the memory cell upon powering up the memory system. These systems generally occupied large amount of surface area in an integrated circuit and are thus considered undesirable. Other devices having included placing a diode on a desired information node of a memory cell and powering the diode to pre-store the desired information bit.
Unbalanced bistable memory cells are also well-known in the prior art having, for example, unbalanced capacitors or resistors connected to a respective information node of a memory cell. Some of these systems require a three level voltage supply wherein the memory cell operates as a non-volatile memory upon transition from a first to a third level and operates as a programmable memory upon transition from a second to a third level. The second voltage level is generally a maintenance level to prevent the unbalance memory cell from assuming the non-volatile state. The voltages are applied along the usual supply line throughout the memory system.